Subsequent Events (Details) - Commercial Paper - ABCP borrowings - USD ($)	Feb. 21, 2018	Feb. 22, 2017
Subsequent Event [Line Items]
Private education loan funding		$ 750,000,000.0
Subsequent Event
Subsequent Event [Line Items]
Private education loan funding	$ 750,000,000.0
Minimum
Subsequent Event [Line Items]
Unused borrowing capacity fee (as a percent)		0.35%
Minimum | Subsequent Event
Subsequent Event [Line Items]
Unused borrowing capacity fee (as a percent)	0.35%
Maximum
Subsequent Event [Line Items]
Unused borrowing capacity fee (as a percent)		0.45%
Maximum | Subsequent Event
Subsequent Event [Line Items]
Unused borrowing capacity fee (as a percent)	0.45%
London Interbank Offered Rate (LIBOR)
Subsequent Event [Line Items]
Basis spread on variable rate		0.85%
London Interbank Offered Rate (LIBOR) | Subsequent Event
Subsequent Event [Line Items]
Basis spread on variable rate	0.85%